The present invention relates to an apparatus for screening a suspension of fibrous cellulose material containing undigested wood.
U.S. Pat. No. 1,185,794 describes a screening apparatus of centrifugal type having a screen consisting of a rotating portion and a stationary portion. The rotatable portion of the screen is constructed from a series of spiders 13 with hubs, rigidly mounted to a driving through shaft 7. The spiders are provided with peripheral portions 15 which are narrower than the hub 14 so that two adjacent spiders 13 are separated by a circumferential opening. Each such opening is closed partially by the inner edge 16 of a stationary, flat ring 17. The stationary portion of the screen consists of a series of such flat rings 17 rigidly mounted in the housing of the screening apparatus by means of a plurality of rods 18. A screening slot or opening is formed between a side wall of each spider 13 located at said peripheral, narrower portion 15, and an opposite side wall of each stationary ring 17. Inside said peripheral portions 15 the rotating spiders 13 are provided with openings 20 through which the fiber suspension to be screened flows in order to reach said screening openings. The screening apparatus according to said patent specification is complicated and thus also expensive to manufacture because of the many spiders and rings which must be alternately fitted extremely accurately one after the other, which is a time-consuming process, so as to form a stationary and a rotating screening portion when finally assembled in the housing. Dismantling the screening portions is equally time-consuming and complicated. The screening gaps or slots are determined from the start by the dimensioning of spiders and rings and the structure does not allow for alteration in one and the same screening apparatus. Neither can the screening gaps be varied in two different screening apparatuses unless two series of spiders and rings with different axial dimensions are manufactured. Furthermore, the structure of the spiders, with a large number of small openings to allow the fiber suspension to pass results in unfavourable flow so that material catches on the many surfaces and edges of the plates, entailing clogging and stops in operation which may be protracted in view of the difficulty in dismantling the spiders and rings one by one. However, the most serious deficiency with the known screening apparatus from the operational point of view is that the screening gaps are charged by knots and other undigested wood in the fiber suspension since the apparatus is of centrifugal type, i.e. the material is fed from the inside, outwards. The knots and other undigested wood are therefore thrown out towards the screening gaps by the centrifugal force, thereby clogging the gaps. In practice, therefore, the screening apparatus described in U.S. Pat. No. 1,185,794 cannot be used for coarse screening to remove coarse reject from a fiber suspension.
The coarse screening apparatus used hitherto for separating coarse reject from digested pulp can only function satisfactorily if the digested pulp is diluted to considerably lower concentrations. The pulp must then usually be re-thickened in the subsequent treatment stage, particularly if pulp of medium consistency (about 6-15%) is utilized.
A uniform fiber line with respect to the fiber concentration is required in the pulp industry so as to avoid, as far as possible, time-consuming and cost-increasing dilution and thickening between the various treatment stages.